1. Field of the Invention
This invention relates to a method and an apparatus for recording and/or reproducing information signals for a plurality of sorts of optical discs having reciprocally different values of track pitch of the recording tracks and hence differential recording densities. This invention also relates to a method for adjusting the aberration of the present optical disc recording and/or reproducing method and apparatus.
2. Description of Related Art
In an optical disc used up to now as a recording medium for information signals, attempts have been made for raising the recording density. For example, in the case of a magneto-optical disc having a diameter of approximately 65 mm, proposals have been made for reducing the track pitch of the recording track carrying the information signals from 1.6 μm to 0.95 μm to increase the recording density by a factor of approximately five.
For recording information signals on the magneto-optical disc, having the reduced track pitch as described above, and for reproducing the recording information signals, the spot diameter of the light beam scanning the recording track formed on the magneto-optical disc needs to be of a smaller value. The reason is that, if the spot diameter of the light beam becomes larger than the track pitch of the recording track, the recording track cannot be tracked correctly, with the result that information signals cannot be recorded or reproduced on or from a desired recording track.
Thus, proposals have also been made for using an optical pickup device provided with a light source designed for radiating a light beam of a shorter wavelength for reducing the spot size of the light beam illuminated on the magneto-optical disc. If the optical pickup device, radiating the light beam of the short wavelength, is used for recording and/or reproducing a magneto-optical disc, the desired recording track cannot be tracked correctly, because the spot diameter of the light beam is markedly smaller than the recording track width, with the result that the information signals cannot be recorded or reproduced correctly.
For differentiating the track pitch values of the recording tracks from one another for ultimately enabling recording and/or reproduction of plural sorts of the magneto-optical discs with different recording densities by a common optical disc recording and/or reproducing apparatus, there has also been proposed an optical disc recording and/or reproducing apparatus including an optical pickup device having a plural number of light sources each radiating a light beam of a shorter wavelength and a light beam of a longer wavelength.
This optical disc recording and/or reproducing apparatus is designed to switch between the plural light sources to radiate light beams of different wavelengths to adapt itself to a plural number of sorts of the magneto-optical discs having reciprocally different values of the track pitch of the recording tracks.
With the magneto-optical disc having the track pitch of 1.6 μm, double refraction is more significant than with a magneto-optical disc having a track pitch of 0.95 μm, and hence astigmatic aberration is produced in an optical system when the light beam is transmitted along an optical path in the magneto-optical disc. So, in the optical disc recording and/or reproducing apparatus, dedicated to a magneto-optical disc having a track pitch of 1.6 μm, the value of this astigmatic aberration is supervised with respect to the entire optical system. On the other hand, with the magneto-optical disc having the track pitch of 0.95 μm, the double refraction is lesser than with the magneto-optical disc having a track pitch of 1.6 μm, such that the astigmatic aberration is suppressed in the optical system